Electrographic impaging and developing processes, for example electrophotographic imaging process and techniques, have been extensively described in both the patent and other literature, for example. U.S. Pat. Nos. 2,221,776, issued Nov. 19, 1940; 2,277,013, issued Mar. 17, 1942; 2,297,691, issued Oct. 6, 1942; 2,357,809, issued Sept. 12, 1944; 2,551,582, issued May 8, 1951; 2,825,814, issued Mar. 4, 1958; 2,833,648, issued May 6, 1958; 3,220,324, issued Nov. 30, 1965; 3,220,831, issued Nov. 30, 1965; 3,220,833, issued Nov. 30, 1965; and many others. Generally these processes have in common the steps of forming a latent electrostatic charge image on an insulating electrographic element. The electrostatic latent image is then rendered visible by treatment with an electrostatic developing composition or developer.
Conventional developers include a carrier that can be either a triboelectrically chargeable, magnetic material such as iron filings, powdered iron or iron oxide, or a triboelectrically chargeable, non-magnetic substance like glass beads or crystals of inorganic salts such as sodium or potassium chloride. In addition to the carrier, electrostatic developers include a toner which is electrostatically attractable to the carrier. The toner is usually a particulate polymeric material which may, if desired, be suitably darkened or colored for image viewing purposes with a colorant such as dyestuffs or pigments, for example, carbon black.
To develop an electrostatic image, the dry developer can be applied imagewise to the electrostatically charged surface by various techniques. One such technique is known as cascade development and is described in U.S. Pat. No. 2,618,552, issued Nov. 18, 1952.
Another suitable developing technique is known as magnetic brush development and is described in U.S. Pat. No. 3,003,462, issued Oct. 10, 1961.
In conventional electrophotographic applications, the developed image is formed on a photoconductive element and is transferred to a receiving sheet. The image thus transferred is then fixed, i.e., made permanent typically by heating to fuse the transferred image. Thus, the toner material must be capable of being fused under temperature conditions which will avoid any charring, burning or other physical damage to the receiver sheet which is typically formed of paper.
A variety of processes and apparatus have been described in the electrographic art for accomplishing fixing of the transferred image. Typically this is accomplished by the combined application of heat and pressure, for example, by bringing the receiving sheet containing the transferred developed toner image into contact with a heated fusing roller. In addition to the use of a heated fusing roller other devices may be utilized for the fixing of the developed toner image such as contacting the developed toner image with a heated platen or some other similar heated member.
The property of a toner to fuse adequately at a low temperature is quite important. This property, described herein by a low "onset of fusing" temperature, allows operation at lower temperature, thus consuming less energy and increasing machine life by reducing the degradation effects of heat on elastomeric fusing roller materials, electronic components, and the like; and further allows higher-speed machine operation.
However, regardless of the type of heated fusing member employed, it has been recognized in the electrographic art that there exists a substantial problem associated with the "off-setting" of individual toner particles of the developed image during the fixing operation. Off-setting is the undesirable transfer of toner particles from the developed toner image carried on a receiving member (e.g., copy sheet) to the surface of the heated fusing member. The surface of the fusing member therefore becomes contaminated with toner particles; and, upon further use of such a contaminated fusing member, it is found that these toner particles adhered to the surface of the fusing member are transferred to subsequent copy sheets or receiving members. As a result, either a ghost image of previously fixed images is formed on subsequent copy sheets, or undesirable deposits of toner material are formed in background areas of subsequent copy sheets, i.e., scumming or discoloration occurs in background areas of subsequent copies.
Thus, a high "hot offset" temperature, i.e., the temperature at which the cohesive strength of the toner matrix material (or binder resin) is lost and the toner thus sticks to the fusing roller and causes offset, is also desirable for a toner. The difference between the "onset of fusing" temperature and the "hot offset" temperature is referred to herein as "offset latitude". The greater the offset latitude is, the wider the temperature range in which the fusing roller can operate.
Although polyester resins have been broadly suggested for use as a matrix material or binder for electrographic toners, it appears that few specific polyester compositions have been described for use in electrographic toners. For instance, U.S. Pat. No. 3,647,696 suggests that polyester condensates like poly(ethylene-co-2,2-dimethyl-1,3-propylene terephthalate-coisophthalate) are useful as binder resins for electrographic toners.
It has generally been difficult to obtain a polyester toner composition that has both a low "onset of fusing" temperature and a high "hot offset" temperature, in other words a wide "offset latitude". One method for accomplishing this desired result is to crosslink the polyester resins as described in U.S. Pat. No. 3,938,992. Crosslinking however can be difficult to control and can introduce other manufacturing problems such as, for instance, difficulty in grinding toner powders, or difficulty in dissolving polyester toner compositions for spray drying, etc. Thus, it would be desirable to obtain a polyester toner composition having the above-described desirable fusing properties without requiring a crosslinked polyester resin.